Exhaust gas emitted from an internal combustion (IC) engine, is a heterogeneous mixture that may contain gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Exhaust gas treatment systems typically include one or more selective catalytic reduction (SCR) devices and a reductant delivery system. The SCR devices include a substrate having a washcoat disposed thereon that operates to reduce the amount of NOx in the exhaust gas. The reductant delivery system injects a reductant solution including an active reductant such as, for example, ammonia (NH3), urea (CO(NH2)2), etc., which mixes with the exhaust gas. When the proper amount of reductant is supplied to the SCR device under the proper conditions, the reductant reacts with the NOx in the presence of the SCR washcoat to reduce the NOx emissions. The quality of the reductant solution may affect the efficiency at which the SCR device effectively reduces the NOx emissions.